<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        window.onload = function () {
            var rate = 3.75;
            wh = document.body.clientHeight || document.documentElement.clientHeight;
            console.log(wh);
            document.documentElement.style.fontSize = wh / rate + "px";
        }
    </script>

</head>
<body>

</body>
</html>